1. Field of the Invention
The present invention relates to a small and lightweight image forming lens system which can be advantageously used with favorable results in a facsimile machine, an image scanner, or a compact camera, etc.
2. Description of Related Art
A conventional image forming lens system includes at least three lens groups, such as a triplet lens system which has three lens groups, each consisting of three lenses, or a Tessar lens system having three lens groups, each of which consists of four lenses.
FIG. 1 shows a known image forming lens system having a lens barrel 1 in which three lens groups L1, L2 and L3 are incorporated. The lens barrel 1 is provided on the inner peripheral surface thereof with a large diameter portion 1a and a small diameter portion 1b. The first lens group L1 is mounted to the large diameter portion 1a and abuts against the second lens group L2. The third lens group L3, an annular spacer 3, and the second lens group L2 are inserted in the small diameter portion 1b of the lens barrel 1 in this order from an object side (i.e., front) of the lens system. These elements abut against each other. The rearmost position of the third lens group L3 is restricted by a flange 1c which is integral with the lens barrel 1 at the rear end thereof. The frontmost position of the first lens group L1 is restricted by a lens holding ring 2 which is threadedly engaged with the lens barrel 1. The annular spacer 3 is provided with a fixed aperture stop (i.e., diaphragm) 3a integral therewith.
However, in the image forming lens system as mentioned above, the lens barrel 1 has a complex internal structure. Furthermore, the spacer ring 3, which also functions as an aperture stop, is complex and expensive.